In the field of internal combustion engines, turbochargers are forced-induction devices that are utilized to increase the pressure of the intake air provided to the engine. Exhaust gases from the engine are routed to the turbocharger and are utilized to drive a turbine wheel. The rotational force generated by the turbine wheel is utilized to drive a compressor wheel, which pressurizes ambient intake air and supplies the pressurized intake air to the engine. By pressurizing the intake air, the amount of air and fuel that can be forced into each cylinder during an intake stroke of the engine is increased. This produces an increased power output relative to a naturally-aspirated engine.
The turbine wheel and the compressor wheel are mounted to a common shaft. The shaft is loaded axially and bears against a thrust washer, which in turn bears axially against the thrust bearing. For example, during operation of the turbocharger, the thrust bearing is axially loaded by the shaft due to force imbalances between the turbine wheel and the compressor wheel (e.g., arising pressure imbalances and wheel geometry). During engine startup, such as during cold startup, this axial loading occurs primarily in one direction against a first side (e.g., axial face) of the thrust bearing, which may be considered a loaded side of the thrust bearing. For example, the axial loading during startup may be in a direction from the turbine wheel toward the compressor wheel, or may be directed from the compressor wheel toward the turbine wheel in some applications. A second side (e.g., axial face) of the thrust bearing that is opposite the first side is loaded to a lesser degree than the first side (e.g., is generally not loaded) during startup and may be considered an unloaded side of the thrust bearing; however, this second side may still experience axial loading thereagainst during engine startup and other operating conditions.
The interface between the thrust bearing and the thrust washer is lubricated by oil (e.g., engine oil). More particularly, the oil is pumped through the thrust bearing and exits loaded and unloaded sides of the thrust bearing (i.e., an axial face that receives the axial load, and an opposite axial face). The oil thereby forms a lubricating film between the loaded and unloaded sides of the thrust bearing and the thrust washer.
During normal engine operation, the oil is heated and, thereby, has a relatively low viscosity, which allows sufficient oil to flow between the loaded side of the thrust bearing and the thrust washer despite there being relatively small axial clearance between the loaded side of the thrust bearing and the thrust washer (e.g., approximately 10 microns in some applications) as compared to the larger axial clearance between the unloaded side of the thrust bearing and the thrust washer or other member (e.g., approximately 100 microns in some application). During engine startup (e.g., cold start), however, the oil may have a relatively low temperature and, thereby, high viscosity. The high viscosity of the oil and relatively small axial clearance between the loaded side of the bearing and the thrust washer restricts the oil from flowing between the loaded side and the thrust washer. Simultaneously, the relatively large axial clearance between the unloaded side and the thrust washer allows a relatively high proportion of the oil to flow between the unloaded side and the thrust washer. For example, during engine startup, oil flow may be distributed between the loaded side and the unloaded side of the thrust bearing at a ratio of 10% to 90%.